A varactor is a type of diode that behaves as a variable capacitor when operated in reverse bias. With more particularity, a varactor includes an np semiconductor junction, wherein capacitance of the junction is a function of voltage applied across the junction. Specifically, the capacitance of the np junction (and thus the varactor) decreases as the reverse voltage across the np junction increases.
Varactors are conventionally designed to conform to a voltage-dependent capacitance response curve, where the curve can be tailored to a specific range of values (e.g., both in voltage and resulting capacitance) as a function of the semiconductor material of the varactor, amplitude of the voltage applied across the np junction, and frequency of the voltage applied across the np junction. Varactors are typically found in electronic equipment where tuning is desired; specifically, a varactor is typically placed in parallel with an inductor to form a resonant frequency circuit, as such they are known as “voltage controlled oscillators” (VCO's). When the reverse voltage across the np junction of the varactor changes, the resonant frequency of the tuning device (the combination of the varactor and inductor) likewise changes. With the inductor value typically constant, the frequency range for the oscillator is dependent and limited by the varactor capacitance to voltage range.
From the above, it can be ascertained that it is desirable to allow for as much flexibility as possible with respect to capacitance values that can be achieved by the varactor, and it is further desirable to cause the varactor to reach a desired capacitance as quickly as possible. Conventionally, however, there are few suitable solutions to allow for the shifting of the voltage-dependent capacitance curve of semiconductor materials when used as varactors.